Cu and SnO₂-Modified Carbon Felt for Electroenzymatic CO₂ Upcycling

This study reports scalable bioelectrodes for sustainable CO₂-to-formate conversion that integrate on-electrode cofactor regeneration with enzyme immobilization. Carbon felt (CF) supports were coated with copper (Cu) or tin oxide (SnO₂) nanoparticles, allowing for reproducible and straightforward fabrication. Electrochemical characterization revealed that Cu-modified electrodes (CF-NpCu) outperformed SnO₂-modified ones (CF-NpSnO₂) in NADH regeneration, achieving nearly double the faradaic efficiency (FE) toward formate and conversion yield. Coupling CF-NpCu electrodes with affinity-immobilized formate dehydrogenase (FDH) produced 4.4 mM formate after 5 h, a threefold increase compared to the free enzyme system. Although the free enzyme displayed higher intrinsic kinetics, immobilization positioned FDH proximal to the electrode, mitigating diffusional limitations, accelerating NADH turnover, and improving stability. The integrated system achieved a productivity of 43 µmol h^-1 cm^-2 and demonstrated reusability, highlighting its practical applicability. Despite moderate efficiency losses due to side reactions such as hydrogen evolution, this work establishes a scalable bioelectrode platform that effectively combines cofactor regeneration with enzymatic CO₂ reduction, providing a promising route toward sustainable and industrially relevant electroenzymatic processes.